Make sure people know--especially early voters that you hear voice their regret--that they can still vote on Election Day. It may be a pain in the ass to vote in person, but if you do it then it still should invalidate any previously cast ballots.

Many states have ways online or by phone to spoil or cancel previous ballots as well and request new ones, so there's still time to do that. Worst case, go down to your county's election office and ask the procedure for changing your vote.